Certain Accounts cannot send Externally?!

i help administrate a small old network, within which we currently have a mail server running mdaemon version 3! (to upgrade to Exchange end of the year). Intermittently, but becoming more frequently, random user accounts stop being able to send external mail. first of all, deleting and recreating the account on the server did the trick, but today this did not work with a user experiencing the same problem. i have recreated several times, as well as restarted mdaemon system service, and rebooted the mail server.

when attempting to send, this appears in the message router window on the server: